1. Field of the Invention
The present invention relates to a direction finding system and a direction finding apparatus for finding a direction from which a radio wave comes. In particular, the present invention relates to a direction finding system and a direction finding apparatus in which both a transmitter and a receiver are movable instead of being stationary, or in which both the transmitter and the receiver are installed on a single mobile unit.
In more detail, the present invention relates to a direction finding system and a direction finding apparatus that allow easy identification of a source of a received signal using a plurality of receiving antennas. In particular, the present invention relates to a direction finding system and a direction finding apparatus in which a plurality of reception sections are very closely spaced and which do not require a synchronization signal specially used for location search.
2. Description of the Related Art
Wireless communication technologies, such as wireless local area networks (LANs), have become widespread in recent years. For example, a wireless LAN feature has been increasingly common not only in information devices such as personal computers (PCs) but also in small and light consumer electronics (CE) devices such as digital cameras, music players, and cellular phones. In many applications of wireless communication, nondirectional antennas are used to achieve flexible connection. In contrast, portable devices having the wireless LAN feature require a small antenna, and in a typical application, two particular devices are held relatively close to each other to achieve connection therebetween using a directional antenna. Such an application requires a technique for accurately determining or estimating a direction from which a radio wave comes or a location of a source of the radio wave. It is also desirable that both a transmitter and a receiver are not stationary but movable, or that both a transmitter and a receiver are installed on a single mobile unit.
For example, a sensor system has been proposed that includes: two signal sources for generating a source signal; two antennas for transmitting the source signal to an object and receiving a reflected signal from the object, and are configured to introduce a phase shift into the source signal or the reflected signal to generate a plurality of signal patterns; and an information processing device programmed to receive the reflected signal and to determine bearing information for the object based on position and phase information in the plurality of signal patterns (see Japanese Patent Laid-open No. 2005-91365, for example). This sensor system allows an object outside a particular range to identify an object within that range, and makes it possible to determine a direction from which an object approaches an automobile.
In addition, a wireless communication system has been proposed that has a delay element for producing a delay shorter than a cycle of a clock signal to generate a delayed clock signal or a delayed response signal. In this wireless communication system, at the time of transmitting a distance-measuring signal, a clock signal and the delayed clock signal, or the clock signal and the delayed response signal, are used to start a counting operation, and based on a count value obtained by the counting operation and the response signal, or the count value and the clock signal, a difference in timing between counting of the counting operation using the clock signal and receipt of the response signal is detected, and a count value at the time of the receipt of the response signal and the detected timing difference are used to calculate a distance between wireless communication devices (see Japanese Patent Laid-open No. 2007-47047, for example).
In general, in the wireless communication, a communication signal radiates from a transmitter in all directions, and location search is accomplished by either 1) detection using signals transmitted from a plurality of transmitters at different locations or 2) detection using a plurality of receivers at different locations.
However, the location search requires a sufficient distance between the devices or use of a synchronization signal specially used for the location search. This involves an increase in cost.
Further, a direction finding apparatus has been proposed that uses a phase difference measuring instrument to measure a difference in phase between a pair of antennas, uses a time difference measuring instrument to measure a difference in time between the antennas, and uses a direction calculation device to calculate a direction in which a signal has come based on the phase difference and the time difference (see Japanese Patent Laid-open No. Hei 6-273504, hereinafter referred to as Patent Document 1, for example). This direction finding apparatus allows two-dimensional direction finding but not three-dimensional direction finding, however.